Since the typical liquid crystal display (LCD) needs backlight to operate and is complicated in fabrication, alternative displays are further developed. Recently, a display by means of organic light-emitting diodes (OLEDs) has been developed due to its self-light-emitting and easily manufactured features. In addition, the OLED panel has advantages of wide viewing angles, low cost, reduced thickness and flexible operational temperature. The OLEDs can be used as pixel units of an active matrix electron luminescent display, and thus the OLED panel is expected to substitute for the LCD in the near future.
FIG. 1 illustrates a conventional driving circuit for driving an OLED pixel. The pixel unit comprises an organic light-emitting diode OLED, two transistors M1˜M2 and a capacitor Cs (so-called as 2T1C). The gate electrode of the transistor M1 is coupled to a scan line 10, and the other two electrodes of the transistor M1 are coupled to a data line 20 and the gate electrode of the transistor M2, respectively. The source and drain electrodes of the transistor M2 are coupled to a source voltage Vdd and the P electrode of the organic light-emitting diode OLED. The N electrode of the organic light-emitting diode OLED is coupled to a ground voltage GND. The capacitor Cs is coupled between the source electrode and gate electrode of the transistor M2.
During operation of the scan line 10, the transistor M1 is switched on. Meanwhile, via the data line 20, a driving voltage is inputted and stored in the capacitor Cs. The driving voltage can also bias the transistor M2 to result in a constant current Id passing through the organic light-emitting diode OLED. The organic light-emitting diode OLED emits light accordingly.
For a purpose of forming the active matrix and its peripheral circuit on the same substrate, a so-called low-temperature polysilicon thin film transistor (LTPS-TFT) technology was developed with improved electrical properties of TFTs and other benefits. However, since the threshold voltage and mobility of such LTPS-TFT vary with manufacturing processes to a certain extent, some problems may occur. For example, under a constant voltage applied to the capacitor Cs, the resulting intensity of current passing through the organic light-emitting diode OLED may be different for the LTPS-TFT manufactured by different processes. The light intensity emitted by the OLED cannot be well expected.